This invention relates to a system for monitoring the relative displacement between one object and another object normally located adjacent to, or in close proximity of, the first unit. In particular, a security system uses an analog signal which may result from a magnetic field, an optical or other source to establish a window within which a movable object such as a door can move with respect to a fixed object such as the door frame without putting the system into an alarm condition; but in which an alarm condition arises should the door move outside the window. The window is a movable window which allows for automatic adjustment over time for variations between the door and frame such as are caused by changes in temperature and humidity as a result of which the physical dimensions of the door and window change. As a further refinement, the window has at least two ranges one of which is a nominal range that pertains during normal situations, and the other of which is created when the system goes into alarm.
Conventionally, security systems monitoring various possible points of entry into a facility used different types of sensors to determine whether, for example, a door is positioned adjacent its associated frame (i.e., closed), or whether the movable portion of a window is adjacent its frame or a fixed portion of the window unit (i.e., closed). Generally, these security systems use a magnetic sensor employing a reed switch or the like, and a magnet. The magnet is positioned on the door or movable portion of the window, and the reed switch on the door or window frame adjacent the magnet.). Essentially, these security or monitoring systems detect the presence, strength, and polarity of a magnetic field as an indication that a static condition (door closed) is present.
A major difficulty in making these installations is achieving the correct "balance". During installation, it has always been the job of the installer to mount the sensor in an optimum position to maximize the "catch" of the sensor, and minimize false alarms. "Catch" relates to that movement of the door relative to its frame is the maximum allowable. False alarms occur when the system is unnecessarily put into alarm (as by the door being moved relative to frame a distance which is an acceptable distance but which is outside that permitted by the particular set-up). Typically the installer monitors the normal/alarm status of the sensor as he moves one portion of the sensor back and forth relative to the other. As he does this, he notes both how close and how far he can move the one portion of the sensor to the other before he gets an alarm indication. Once he has this information, he then attempts to mount the portion of the sensor he has been moving in the middle of the two points at which an alarm would occur. A number of factors determine how "balanced" the resulting installation is. One is the competency of the installer. Another is environmental effects for which it is difficult for the installer to account. If the installation is made on a hot, humid day, the center position which the installer locates is probably going to be different than if the installation is made on a cool, dry day. What is necessary is a security system which automatically adjusts for both improper or inaccurate installation, as well as environmental effects to achieve a "balanced" sensor installation.